The present invention relates to differential amplifiers and more particularly to differential amplifiers constructed using transistors having bipolar operating characteristics so that the differential amplifier is compatible with standard complementary MOS (CMOS) technology.
In the present development of integrated electronic circuits, there is increasing interest in the possibility of obtaining analog functions and digital functions on the same circuit. Although bipolar technologies are known to be better for purely analog circuits, MOS technologies are preferred for the implementation of the digital part of a circuit. While many attempts have been made to combine the advantages of both bipolar and MOS technologies, the same involve either the modification of existing manufacturing processes or the production of devices with very limited applications.
By way of example, such processes which are modified to permit the integration of bipolar and MOS devices are described in the article entitled "CMOS and Complementary Isolated Bipolar Transistor Monolithic Integration Process" by M. Darwish and R. Taubenest in the Journal of the Electrochemical Society, Volume 121, No. 8, August 1974, and the article entitled "Bimos Micropower IC's" by Otto H. Shade, Jr. in the IEEE Journal of Solid-State Circuits, Volume SC-13, No. 6, December 1978. The modifications described in those articles consist of supplementary stages of manufacture which increase costs and reduce the yields of the circuits produced by such processes.
There has also been proposed the production of bipolar devices with MOS technology as particularly described in the article entitled "A CMOS Voltage Reference" by Yannis P. Tsividis et al. in the IEEE Journal of Solid-State Circuits, Volume SC-13, No. 6, December 1978, and the article entitled "A Low-Voltage CMOS Band-Gap Reference" by Eric A. Vittoz et al in the same IEEE Journal of Solid-State Circuits, Volume SC-14, No. 3, June 1979. Such bipolar devices as are described in the above articles are often called a "MOS substrate transistor" and is more particularly shown in FIG. 1 of the drawings. As will be appreciated, the applications of such devices are limited.
There are still other known examples of devices wholly compatible with standard MOS technolgy that have the characteristics of a bipolar transistor without having the limitations of the MOS substrate transistor of FIG. 1. Such an example is shown in FIG. 2, as will be described in greater detail below. Such devices as are shown in FIG. 2 define a bipolar transistor with a current gain .alpha. which will always be lower than unity, but with a current gain .beta. which may attain quite high values thereby making the device perfectly usable in practice. However, although such devices may be used satisfactory in certain applications instead of conventional bipolar transistors, there are other applications in which the low and poorly controlled value of the .alpha. current gain prevents the use of conventional layouts of bipolar circuits.
By way of example, this is particularly the case when it is desired to implement an integrated differential amplifier with the aid of such devices. In such instances, it is to be understood that the differential amplifier is defined to be a circuit having a pair (differential pair) of transistors having emitters connected to a current source and collector currents defined by the signals applied to the bases of the transistors. In some instances, such circuits are currently used to amplify the difference between two signals, in which case the output quantity is a voltage. In other instances, the circuits are used to achieve a "current switching" in which case the output quantity can be a current.
Accordingly, the present invention has been developed to provide an improved differential amplifier which can be produced using transistors having bipolar operating characteristics and compatible with standard CMOS technology.